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Robot Design and Kinematics

In the basement of Latrobe Hall “lives” a mechanical arm that, purely through pneumatic on/off switches operating pistons on a series of joints, can maneuver a tool into any pre-programmed position. An algorithm translates the user’s input of a spatial coordinate into a series of on/off switches for the pistons on the arm that take it to that coordinate. This “superarm,” designed by Professor Gregory Chirikjian, can perform highly repetitive tasks efficiently and cheaply, even fairly complicated ones involving a series of movements, such as placing parts in an assembly line.

For situations in which the motion desired by the robot cannot be programmed ahead of time, engineers depend on motors that can provide as many degrees of freedom as possible. One degree of freedom means that the motor can move, say, up and down; two degrees means that it can go up and down and side to side, and so on. In each joint of a robotic arm there is a motor, and the more degrees of freedom that motor has, the more general its movement can be. Consider the amazing apparatus that is our shoulder joint —its range of motion is phenomenal. Attach it to the highly articulated elbow and wrist joints, and the human hand becomes a truly miraculous tool, able to reach with ease in any place or any direction. Engineers have good reason to want to mimic this range of motion robotically. The closest they have come is the “spherical” motor, a motor that can rotate in a sphere around any axis. Prof. Chirikjian and his student David Stein have recently built a spherical stepper motor. The moving portion of the motor is a hollow plastic sphere in which magnets have been placed in a regular pattern. This sphere is placed in a cap containing several soft iron cores that are polarized to form a magnetic field. The whole arrangement looks a bit like an egg (albeit a spherical egg) in an egg cup. A current is run through the coils in the cap, changing the magnetic field, and the plastic sphere moves in response. Although Chirikjian’s group is one of many working on the design of a spherical motor, theirs has distinct advantages. In most current spherical motors, the “egg-cup” part of the motor has to envelop the rotor, whereas Chirikjian’s cap is less than a hemisphere and thus provides a greater degree of freedom of movement.

Spherical motors would fill engineering needs in many important applications. Robotic wrist, elbow, and shoulder actuators might be used in small spaces where a human arm would not fit, such as in surgical procedures, or in uninhabitable environments. For a computer to “see,” a camera (its “eye”) would have to operate like our eyes, smoothly tracking an object as it moves within the field of vision. A roaming robot with three spherical-motor wheels under it becomes “omni-directional.” An object placed on a platform made up of an array of spherical motors could move smoothly in any direction.